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List:       koffice-devel
Subject:    Color models and Krite
From:       James Richard Tyrer <tyrerj () acm ! org>
Date:       2007-10-12 1:21:10
Message-ID: 470ECC06.3060401 () acm ! org
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First I note from the documentation:

The CMYK color model
   CMYK is the abbreviation for Cyan, Magenta, Yellow, blacK (although
   officially the K stands for Key, black is much more commonly used).
   This color model is based on ink: a color is specified by the amount
   of ink needed for a point to be perceived as having that color.
   Since CMYK colors are used by printers while RGB colors are used
   on-screen, one often wants to convert RGB colors to CMYK colors. As
   this cannot always be done correctly, printed images may turn out to
   look quite different than what is perceived on-screen.

First, I wish to reiterate that this sentence:

	Since CMYK colors are used by printers while RGB colors are used
	on-screen, one often wants to convert RGB colors to CMYK colors.

is not correct.  Print drivers use RGB as input and they convert this to 
CMYK.

It would be clearer if you first explained that CMY color model which is 
based on subtractive color.  That is you shine light on or through 
something and it produces the colors by filtering out (subtracting) the 
part of the color which isn't wanted.  And then explained that CMYK is 
based on 4 color printing (with ink) where the four images are placed 
one on top of the other.

The problem in printing is not the RGB to CYMK conversion, the main 
problem is that the color space of a 4 color printed image is much 
smaller that the sRGB space (which is the presumed color space of a 
monitor).  The image illuminated by reflected light on white paper that 
doesn't reflect 100% has a smaller color gamut -- that is the issue. 
There is no difference between colors produced with perfect colors in 
RGB and CMY (you could also add Grey filters and call this CMYK).  This 
is a basic principle of (photographic) color printing.  A secondary 
issue in printing is color failure.  Color failure is a fancy term to 
indicate that the color in ink or dye are not perfect.

Krita has now reached the point where it can be used.  So, I have tried 
it out.  I find out that the color management system is wrong.  What is 
referred to in the USA as Bass Akwards.

I loaded my color test targets.  And tired a few things.

First, I find that I can not set the Printing Color Model.  Despite 
several attempts, it remains set at: CMYK (8 bit integer/channel) 
despite several attempts.  Since most users will probably be printing to 
their desktop inkjet printer, CMYK is a *POOR* choice for a default 
since the printer drivers need RGB input.

I tried converting the displayed image to a different color image.  I 
used Convert Layer Type to convert to Adobe CMYK (8 bit).  With the 
default: Perceptual since I don't think that there is any white point 
issues here.  Actually, I'm not sure exactly what the choice of 
Rendering Intent here would do.  Does it override the default set in the 
KCM?  If so, I would change the KCM slightly.

The colors of the image on the screen changed which is correct and is 
what I would expect in Adobe PhotoShop.  This happens not because of the 
conversion from RGB to CMYK by because Adobe RGB has a larger color 
gamut than Adobe CMYK.

Then I saved the image as PNG.  The thumbnail didn't look quite correct 
so I opened it in KolourPaint (which IIUC doesn't have color 
management).  The colors are not the same.  This is WRONG.  Changing the 
color profile changes what the data represents, you do NOT change the data.

Apparently what is happening is that when the color profile of the image 
is different than that of the screen that a Save To results in the 
saving of the modified data which is being displayed on the screen 
rather than the actual data (before it is modified to be displayed).

Is it possible to add a color profile for the Fuji Frontier 
printer/Fujicolor paper combination.  This is what you usually get if 
you order actual photographic prints from a one-hour place.  Commercial 
labs probably have a Durst printer which would be slightly better, but 
having something for photographic printers would be nice.

-- 
JRT

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